Reductive Catalytic Fractionation of Abies Wood into Bioliquids and Cellulose with Hydrogen in an Ethanol Medium over NiCuMo/SiO₂ Catalyst

Abstract

Noble metal-based catalysts are widely used to intensify the processes of reductive fractionation of lignocellulose biomass. In the present investigation, we proposed for the first time using the inexpensive NiCuMo/SiO₂ catalyst to replace Ru-, Pt-, and Pd-containing catalysts in the process of reductive fractionation of abies wood into bioliquids and cellulose products. The optimal conditions of abies wood hydrogenation were selected to provide the effective depolymerization of wood lignin (250 °C, 3 h, initial H₂ pressure 4 MPa). The composition and structure of the liquid and solid products of wood hydrogenation were established. The NiCuMo/SiO₂ catalyst increases the yield of bioliquids (from 36 to 42 wt%) and the content of alkyl derivatives of methoxyphenols, predominantly 4-propylguaiacol and 4-propanolguaiacol. A decrease in the molecular mass and polydispersity (from 1870 and 3.01 to 1370 Da and 2.66, respectively) of the liquid products and a threefold increase (from 9.7 to 36.8 wt%) in the contents of monomer and dimer phenol compounds were observed in the presence of the catalyst. The solid product of catalytic hydrogenation of abies wood contains up to 73.2 wt% of cellulose. The composition and structure of the solid product were established using IRS, XRD, elemental and chemical analysis. The data obtained show that the catalyst NiCuMo/SiO₂ can successfully replace noble metal catalysts in the process of abies wood reductive fractionation into bioliquids and cellulose.

1. Introduction

Among the countries with large areas of forest, lignocellulose waste generated during the processing of wood is the main renewable source of raw material for the production of various chemical products and biofuels [1,2].

Currently, one of the largest-scale areas of wood waste recycling is its use for energy purposes, in particular for the production of solid fuels such as pellets and briquettes [3]. However, due to its unique chemical composition (cellulose, hemicelluloses, lignin), wood biomass can be used to produce many valuable substances that are alternatives to petrochemical synthesis products [4,5,6,7].

Bioliquids, due to their large content of methoxyphenols, are an alternative raw material for the production of motor fuel components and various chemical products [8,9]. Monomeric alkylmethoxyphenols are platform molecules on the basis of which such valuable products as anticancer and antimicrobial drugs can be produced [9,10].

In recent years, research aimed at creating high-tech methods of processing lignocellulose biomass (including wood and agricultural waste), providing full use of the main components of biomass, has been intensively undertaken. Promising areas of research are focused on the development of effective methods of fractionation of the main components of lignocellulose biomass [8,11,12].

The most promising are the methods of catalytic fractionation of lignocellulose biomass [13,14]. To obtain monomeric phenolic compounds from the lignin component of biomass, reductive catalytic fractionation (RCF) methods have been proposed [15,16,17,18]. During the RCF process, lignin and hemicelluloses are depolymerized, while the main part of the cellulose is preserved [19].

RCF processes are carried out under harsh conditions (high temperature, pressure, acidity of the reaction medium). Thus, there are special requirements on catalysts for the RCF process. The catalysts must have a large pore size, remain stable under hydrothermal conditions, and not be deactivated by process by-products (for example, organic acids) [20]. In the literature, there are a few examples of the use of solid catalysts based on noble and non-noble metals supported on mesoporous oxide and carbon carries in the RCF processes of lignocellulose biomass [21,22,23,24].

In the process of birch wood RCF at a temperature of 250 °C, H₂ pressure of 3 MPa and duration of 3 h, the catalyst Ru/C increased to 48 wt% yield of monomeric phenols, among which 4-propyl syringol and 4-propyl guaiacol predominate [21]. Almost the same yield of a mixture of monomeric phenols (51 wt%) in which propanol-substituted methoxyphenols predominate was achieved via the birch wood RCF process under similar conditions using the catalyst Ni/Al₂O₃ [23]. High (51 wt%) mixture of monomeric phenols with increased content of 4-propanol syringol and 4-propanol guaiacol was achieved in the process of beech wood RCF in the presence of catalyst Ni/C (temperature 200 °C, H₂ pressure of 2 MPa, methanol–water medium) [25].

Analysis of the literature data showed that the nature of the metal has a significant effect on the yield and composition of RCF products [20]. Catalysts based on the non-noble metals are active at higher temperatures than Ru-, Pt- and Pd-containing catalysts. The reductive depolymerization of lignin in the presence of ruthenium catalysts proceeds with the predominant formation of propyl-substituted methoxyphenols. Nickel-containing catalysts promote the formation of propanol-substituted methoxyphenols.

The actual problem is the development of inexpensive and effective catalysts based on non-noble metals that are stable under hydrothermal process conditions for RCF processes of lignocellulose biomass.

There are examples of the use of nickel-containing catalysts for hydrogenation of lignin and wood [23,25,26]. In particular, the catalyst Ni/C shows activity comparable with catalysts Pt/C, Pd/C, Ru/C in organosolv lignin depolymerization in supercritical butanol with the obtaining of alkylated phenols [27]. Additionally, it has been shown [28] that Ru/C and Ni/C catalysts provide the close values of the product yield (~25 wt%) from corn stover lignin.

Since inexpensive Ni-containing catalysts are more promising for practical use, they must be chemically promoted with molybdenum and other compounds to enhance stability of the active component with respect to the reaction medium [27,29,30,31].

Ni–Cu catalysts have been found to be more attractive for the HDO than monometallic Ni catalysts [32]. Cu is applied as a promoter, primarily to reduce the reduction temperature of Ni, and to prevent excessive carbon deposition on Ni [33]. Recent works have shown that Ni–Cu bimetallic supported catalysts are highly active in catalytic hydrotreatment of bio-oil and its model compounds. Their activity is comparable to noble metal-based catalysts and better than commercial sulfited catalysts [32,33]. Modification of nickel catalysts with molybdenum leads to a decrease in the coke formation in the guaiacol hydrodeoxygenation process [34].

The NiCuMo/SiO₂ catalyst, developed for the treatment of bio-oil, has a high activity in hydrodeoxygenation of guaiacol, which is a structural fragment of softwood lignin [34].

Previously, we showed [35,36] that the NiCuMo/SiO₂ catalyst more than doubles the yield of liquid products formed during the thermal conversion of acetone lignin and ethanol lignin from aspen wood at 300 °C in butanol and ethanol media. The catalysts NiCu/SiO₂ and NiCuMo/SiO₂ are also active in the hydrogenation of cellulose into sorbitol and 5-HMF [37].

It is well known that the chemical composition of lignins of softwood and hardwood woods differs significantly. Softwood lignin is predominantly composed of phenylpropane units of the guaiacyl type, while syringyl-type structural units predominate in hardwood lignin [38].

By hydrogenation of pine wood in the presence of Pd/C catalyst at 195 °C in a dioxane–water medium, 4-propanolguaiacol was obtained with a yield of 20 wt% [39]. The use of bimetallic Pd/Zn/C in the process of pine wood hydrogenation made it possible to obtain propyl guaiacol with a selectivity close to 100% [40].

Therefore, for the selective production of monomeric derivatives of guaiacol by catalytic depolymerization of lignin, softwood lignin should be used. Abies, pine and larch are the most common types of softwood tree in Eastern Siberia. Abies wood, unlike pine wood, contains few resinous substances that complicate the catalytic processing of wood. Compared to larch wood, abies wood has a lower hemicellulose content and has a looser structure, which facilitates its chemical processing. For this reason, abies wood was chosen in this study as a feedstock for the process of RCF.

In our previous work [36], we performed reductive fractionation of aspen wood lignin in bioliquids enriched with phenolic monomers and cellulose product in ethanol medium at 250 °C, hydrogen pressure 4 MPa, using NiCuMo/SiO₂ catalyst. Ethanol was used as a solvent, since it is a low-toxic solvent and can be produced in the required quantities from biomass using existing industrial technologies [41]. In the present work, we first proposed to use NiCuMo/SiO₂ catalyst to replace more expensive Ru-, Pt-, Pd-containing catalysts in the reductive fractionation of abies wood into bioliquids and cellulose. It was established that this catalyst provides the efficient depolymerization of abies wood lignin to monomer and dimer phenol compounds under hydrogenation conditions that are optimal for the hydrogenation of aspen wood [36].

2. Results

2.1. Thermochemical Properties of Abies Wood

Thermogravimetric studies of thermochemical properties of abies wood were performed in order to select the optimal temperature of catalytic hydrogenation of the wood. Figure 1 represents the TG and DTG profiles of abies wood pyrolysis in an argon atmosphere. The whole pyrolysis process can be divided into three stages, since every single slope change on a TG curve indicates the beginning of a new stage. By considering the TG/DTG curves obtained at the rate of temperature rise 10 °C/min, the following observations and/or comments can be made:

The first stage starts at 50 °C and finishes at 222 °C. The mass loss at this stage is 3.2%, and can be attributed to the removal of water. It is also possible that some light volatile compounds are removed. Stage I is represented by the weak peak on the left-most side of the DTG curve.

The second stage starts at 222 °C and finishes at 400 °C. As the temperature is elevated to 400 °C, the rate of mass loss increases, mainly due to the thermal decomposition of hemicelluloses and amorphous cellulose [42]. This step of thermal decomposition of lignocellulose is known as the zone of active pyrolysis [43], where the main mass loss (62.2%) occurs. Deconvolution of the DTG curve (Figure 2) allows two apparent peaks to be identified at 326 °C and 359 °C, which are assigned to decomposition of hemicelluloses and amorphous cellulose [42]. Merging of these peaks forms a wide peak in the DTG curve starting from 222 °C and having its maximum at 356 °C (Figure 1). The maximum mass loss rate is 9.07%/min at this stage.

The third stage starts at 400 °C and continues to 800 °C, and it can be seen as a tail in both TG and DTG curves. The mass loss at this stage is 10.5%. Stage III can be referred to as the passive pyrolysis stage. The residual mass at the end of the overall pyrolysis process is determined to be 23%. In this temperature range, the dominant thermochemical process is the decomposition and conversion of lignin [44]. A slight weight loss (10.51%) may be due to the aromatization of lignin. It is known [45] that the thermal decomposition of lignin occurs gradually in the temperature range from 220 to 800 °C, which manifests itself on the DTG curve in the form of a wide low-intensity peak. Therefore, it is difficult to determine the ranges of thermochemical conversion of lignin in the wood more accurately.

2.2. Kinetic Analysis of Abies Wood Pyrolysis

The kinetic studies of wood pyrolysis were conducted using the TGA data represented in Figure 1. The temperature range studied was from 200 to 300 °C, which corresponds to the initial stage of the thermal decomposition of lignin. The calculations were performed using the model-fitting Coats–Redfern method. The following equation was used for calculations:

$$\ln \left(\frac{\mathrm{g}\left(\mathsf{\alpha }\right)}{{\mathrm{T}}^2}\right)=\ln \left(\frac{\mathrm{AR}}{\mathsf{\beta }\mathrm{E}}\right)-\frac{\mathrm{E}}{\mathrm{RT}}$$

(1)

where $\mathsf{\alpha }=\frac{{\mathrm{m}}_{\mathrm{i}}-\mathrm{m}}{{\mathrm{m}}_{\mathrm{i}}-{\mathrm{m}}_{\mathrm{f}}}$ is the conversion of a substance; m_i and m_f are the initial and final weights of the substance, respectively; m is the weight of the substance at a measurement point; T is temperature (K); A is the pre-exponential factor (s⁻¹); β is the heating rate (deg/min); E is the activation energy (J/mol); R is the universal gas constant (J/(mol K)); and g(α) is a mathematical model of a dimensionless kinetic function.

The graphic interpretation of Equation (1) for determining the kinetic parameters of the thermogravimetric curves of a first-order reaction uses the equation

$$\ln \left(-\frac{\ln \left(1-\mathsf{\alpha }\right)}{{\mathrm{T}}^2}\right)=\ln \left(\frac{\mathrm{AR}}{\mathsf{\beta }\mathrm{E}}\right)-\frac{\mathrm{E}}{\mathrm{RT}}$$

(2)

The dependence of $\ln \left(-\frac{\ln \left(1-\mathsf{\alpha }\right)}{{\mathrm{T}}^2}\right)$ on 1/T is approximated by a linear function. The validity of the chosen model is evaluated by the square of the correlation coefficient, which characterizes the degree of the linear correlation of chosen quantities. If the experimental and theoretical data coincide, the square of the correlation coefficient is 1. The activation energy is found from the slopes of the obtained straight lines.

A graphical interpretation of the kinetic analysis of pyrolysis is shown in Figure 3. As can be seen, in the temperature range from 200 to 300 °C, the thermal decomposition of wood takes place in two stages. The first stage is characterized by low activation energy −3 kJ/mol. At the second stage, there is a significant increase in the activation energy to 44 kJ/mol. The determination coefficients (R²) were higher than 0.92, indicating that the experimental data showed a good linear relationship. It is likely that when heated, the mechanisms of thermal decomposition of wood biomass change at 250 °C (i.e., the inflection point). The found temperature was used as being optimal for catalytic hydrogenation of abies wood.

2.3. Hydrogenation of Abies Wood in Ethanol Medium

Previously, in [36], the optimal conditions for aspen wood hydrogenation in ethanol in the presence of a NiCuMo/SiO₂ catalyst were determined: 250 °C, 9 MPa, H₂ process time 3 h. These conditions were chosen for comparative examination of the yields and compositions of the products of noncatalytic and NiCuMo/SiO₂ catalyzed hydrogenation of abies wood.

Non-catalytic hydrogenation of abies wood in ethanol at 250 °C gives 36.0 wt% of liquid products, 48 wt% of the solid product, 4.8 wt% of gaseous products (

Table 1. Yield of products of abies wood hydrogenation in ethanol (250 °C, 9 MPa operation pressure, 3 h).

Catalyst	Yield, wt%
	Liquid Products	Solid Products	Gases
			CO	CO2	CH4	Total
No catalyst	36.0	48.0	0.2	1.5	3.1	4.8
NiCuMo/SiO2	42.0	39.5	7.4	2.6	1.8	11.8

). The use of the NiCuMo/SiO₂ catalyst for hydrogenation of abies wood increases the yields of liquid and gaseous products up to 42.0 and 11.8 wt%, respectively, and to a decrease in the yield of the solid product down to 39.5 wt% (Table 1). Obviously, the yield of the liquid products increases owing to intensified reactions of depolymerization of native lignin and hemocelluloses in the presence of NiCuMo/SiO₂, which accelerates reactions of cleavage of ether and C–C bonds [46].

According to the data of the elemental analysis, atomic O/C ratio is lower in the liquid products of non-catalytic and catalytic hydrogenation of abies wood than in the original wood (Figure 4). The most considerable decrease in oxygen content is observed in the liquid products of catalytic hydrogenation of the wood. The atomic ratios O/C and H/C are lower in the solid products than in the original wood (Figure 4).

The results obtained indicate that the reactions of lignin and hemicellulose depolymerization and of resulted liquid products hydrodeoxygenation proceed during abies wood hydrogenation at 250 °C in ethanol medium.

To isolate the catalyst from the reaction mixture, we used the difference in specific density between the NiCuMo/SiO₂ catalyst and cellulose product. The catalyst was isolated by multiple washing with distilled water and decanting until a clear supernatant was obtained (typically, 95% of the catalyst initial mass was recovered). To compensate for the loss of the catalyst during recovery, two identical experiments were conducted, and the catalyst recovered from the corresponding runs was combined, allowing the subsequent run to use exactly the same loading of the starting substances. The recovered catalyst was loaded into the reactor with a new dose of biomass, and the catalytic cycle was repeated at the same working temperature. A total of four consecutive runs at a given temperature with one catalyst sample were conducted. It follows from Figure 5 the yield of liquid products decreased very slightly during four catalytic runs.

The yield of liquid products after the fourth run was reduced by only 4 wt%.

2.4. Composition of Liquid Products of Abies Wood Hydrogenation

To study the molecular weight distribution of liquid products of hydrogenation of abies wood, we used a gel permeation chromatography method. Oligomers, dimers, and monomers formed as a result of partial depolymerization of native lignin were identified in liquid products of non-catalytic hydrogenation (Figure 6). The profiles of the GPC curves of the catalytic hydrogenation shifted towards low-molecular compounds. An apparent increase in intensity of the peak of phenol monomers argues for intensification of the reaction of depolymerization of lignin in the presence of the nickel-containing catalyst to form monomers and dimers [35,47]. It is known from literature data that hydrogen favors stabilization of intermediate products of biomass hydrogenation and prevents them from repolymerization reactions [19,23].

The mass-average molecular weight of 1870 Da is characteristic of the liquid products of non-catalytic hydrogenation of abies wood (

Table 2. Molecular mass characteristics of liquid products of abies wood hydrogenation (250 °C, 9 MPa, 3 h).

Catalyst	Mn a (Da)	Mw b (Da)	PD c
No catalyst	620	1870	3.01
NiCuMo/SiO2	510	1370	2.66

^a Number-average molecular weight; ^b mass-average molecular weight; ^c polydispersity.

). The rather high polydispersity (3.01) indicates the presence in liquid products of a wide variety of molecules (from lignin oligomers to monomer compounds).

Since the monomer and dimer content in the liquid products increased considerably in the presence of the NiCuMo/SiO₂ catalyst, this led to a decrease in their mass-average molecular weight to 1370 Da and in their polydispersion to 2.66 (Table 2). A similar effect of nickel-based catalysts in catalytic hydrogenolysis of organosolvent lignins was observed [35,47]. However, the depth of lignin depolymerization, reflecting the yield of phenol monomers, was somewhat lower for NiCuMo/SiO₂ than in the presence of ruthenium-based bifunctional catalysts [16,18,48].

The GC-MS data show that alkyl derivatives of guaiacyl-type methoxyphenols predominate in the liquid products of abies wood hydrogenation (

Table 3. Yield of monomer phenol compounds produced by non-catalyst and catalytic hydrogenation of abies wood in ethanol (250 °C, 9 MPa, 3 h).

RT	Compound	Structure	Yield, wt% *
			No Catalyst	NiCuMo/SiO2
18.2	guaiacol		1.5	0.4
21.7	4-methylguaiacol		0.3	0.7
24.4	4-ethylguaiacol		2.2	4.7
26.8	4-propylguaiacol		1.3	11.9
29.0	4-propenylguaiacol		2.8	trace
33.9	4-propanolguaiacol		0.8	16.1
Other monomer compounds			0.7	1.9
Dimer compounds			0.1	1.1
Total yield			9.7	36.8

* Per mass of lignin.

). A total yield of monomer alkyl derivatives of methoxyphenols in the liquid products of the non-catalytic hydrogenation is 9.7 wt%. In the presence of NiCuMo/SiO₂ catalyst, their yield is 3.5 times higher (up to 36.8 wt%). The observed yields of monomer compounds are as high as four times of those upon hydrogenation of abies ethanol lignin (9 wt%) under identical conditions [20]. The low yield of monomer alkyl derivatives of methoxyphenols during hydrogenation of ethanol lignin is accounted for by the fact that it contains smaller amounts of reactive β-O-4 bonds, and it has a more condensed structure than the native lignin of abies wood. A fragment of the chromatogram is shown in Figure S1.

4-Propenylguaiacol predominates among the monomer compounds formed during non-catalytic wood hydrogenation. In the liquid product of catalytic hydrogenation, there only trace amounts of 4-propenylguaiacol are present, but the content of 4-propylguaiacol is increased 9-fold owing to intensification of the reaction of hydrogenation of 4-propenylguaiacol to 4-propylguaiacol.

The content of 4-propanolguaiacol also increases (up to 16.1 wt%) in the presence of the catalyst. 4-Propanolguaiacol can be formed via hydrogenation of C=C bonds of the intermediate compound (coniferyl alcohol) [49]. From literature data [23,25], 4-propanolguaiacol can be formed in the presence of Ni-containing catalysts during hydrogenation of lignin by the scheme shown in Figure 7.

However, the presence of Lewis acid centers in nickel-containing catalysts favors removal of γ-OH groups in substituted phenols and results in shifting the reaction towards the formation of propyl-substituted methoxyphenols [50].

During depolymerization of lignins in the presence of molybdenum catalysts, propenyl and propyl derivatives of methoxyphenols are mainly formed [51]. In the presence of NiMo/γ-Al₂O₃ catalyst propyl-substituted methoxyphenols predominate in the composition of monomeric products of lignin depolymerization [52]. We observed the formation of 4-propylguaiacol with sufficiently high yield (11.9 wt%) in the presence of NiCuMo/SiO₂ catalyst. It is probable that the Lewis centers in molybdenum oxide facilitate the removal of γ-OH group to form 4-propylguaiacol (Figure 7).

The data obtained demonstrate the influence of the nature of the catalyst on the composition of phenol compounds formed during lignin hydrogenation. The use of Ru-containing catalysts results in the predominant formation of propyl-substituted phenol compounds [21,49], while Ni-containing catalysts favor the formation of propanol-substituted methoxyphenols.

In addition, liquid products of catalytic and non-catalytic hydrogenolysis of abies wood were investigated using the GC-MS method (

Table 4. Group composition of liquid products of hydrogenation of the carbohydrate part of abies wood in ethanol medium at 250 °C.

Compound	Yields, wt% *
	No Catalyst	NiCuMo/SiO2
Furan derivatives	1.8	4.1
Methyl esters of oxo and hydroxy acids	1.4	0.1
Alcohols and ketones	1.2	0.3
Total yield	4.4	4.5

* Per mass of carbohydrate part of abies wood.

), in which compounds formed from the polysaccharide part were identified.

Under these conditions, products of carbohydrate depolymerization, as well as their subsequent hydrogenation and hydrogenolysis, such as furan derivatives [53] and polyols [18], can be formed.

These products include furan derivatives (2-Furanmethanol, 2-Ethoxytetrahydrofuran); ethyl ethers of oxy and hydroxy acids (lactic acid ester, acetic acid ester), alcohols and ketones (propylene glycol, 2-Hydroxy-3-methyl-2-cyclopentenone). In the presence of a catalyst, the total yield of these products reaches 4.5 wt% of the weight of the carbohydrate part of the wood.

2.5. Composition and Structure of Solid Products of Abies Wood Hydrogenation

IR-spectroscopy, XRD and chemical analytic techniques were used for characterization of solid products of abies wood hydrogenation. The content of cellulose in the solid product of catalytic hydrogenation is 73 wt%, which is greater than that in the solid product of non-catalytic hydrogenation (67 wt%) (

Table 5. Composition of cellulose products and conversion of main components of abies wood during hydrogenation in ethanol (250 °C, 9 MPa, 3 h).

Catalyst	Content, wt%			Conversion, wt%
	Cellulose	Hemicelluloses	Lignin	Cellulose	Hemicelluloses	Lignin
No catalyst	66.5	3.0	30.5	30.1	99.0	42.0
NiCuMo/SiO2	73.2	2.4	24.4	40.0	99.2	70.2

). We showed previously that about the same cellulose contents (70 and 77 wt%) in the solid products of abies wood hydrogenation were observed under identical conditions in the presence of more expensive catalysts, Ru/C [54] and Pt/ZrO₂ [16], respectively. Comparable contents of lignin in the solid products of the catalytic hydrogenation were observed in the presence of catalysts NiCuMo/SiO₂, Ru/C, and Pt/ZrO₂ (24.4, 21 and 27 wt%, respectively).

Data on the composition of products of conversion of main constituents of abies wood during hydrogenation are shown in Table 5. Conversion of hemicelluloses during non-catalytic and catalytic hydrogenation of abies wood at 250 °C reaches 99.0 wt%. The NiCuMo/SiO₂ catalyst increases the conversion of lignin (from 42.0 wt% to 70.2 wt%), and only slightly increases the conversion of cellulose.

IR spectroscopy and XRD techniques were used for characterization of solid products of abies wood hydrogenation.

IR spectra of solid products of non-catalytic and catalytic hydrogenation of abies wood were compared with the spectrum of wood (Figure 8). In the former, there are more intense absorption bands (a.b.) at 3700–3100 cm⁻¹, assigned to stretching vibrations of OH-groups and more intense a.b. at 1033, 1058, 1104 cm⁻¹ characteristic of hydroxyl groups of primary and secondary alcohols and of hydroxyl groups of glucopyranose ring [55]. The increased intensities of these bands are caused by an increase in the content of cellulose in the solid products compared to the wood.

In the IR spectrum of the abies wood, an absorption band at 1735 cm⁻¹ assigned to stretching vibrations of C=O in the ester group of uronic acids in hemicelluloses [56] is observed. The spectrum of the solid product exhibits lower intensities of a.b. at 1606, 1510 cm⁻¹, 1269 cm⁻¹ assigned to lignin [57]. These results argue for lower contents of hemicelluloses and lignin in the solid products of wood hydrogenation compared to the initial wood. The band at around 1420–1430 cm⁻¹ is associated with the crystalline structure of the cellulose, while the band at 896 cm⁻¹ is related to the amorphous region in cellulose [58,59].

In the XRD patterns of the solid products of abies wood hydrogenation, there are two peaks with maxima at 2θ equal to 22.5° and 16° (Figure 9), which relates to the reflections of atoms at the (002) plane and superimposed reflections of atoms at (101) and (10$\stackrel{-}{1}$) planes of the cellulose crystal lattice [60].

The width of XRD lines depends on the size of crystallinity regions referred to as coherent scattering regions. An apparent peak at the diffraction angle 2θ at ca. 22.5° is the criterion of cellulose crystallinity and characterizes the proportion of densely packed cellulose molecules. Higher crystallinity indices are characteristic of the solid products of the catalytic hydrogenation against those of the initial wood (

Table 6. Crystallinity indices of abies wood and of solid products of the wood hydrogenation.

Sample	Crystallinity Index (XRD)
Abies wood	0.54
Solid product of non-catalytic hydrogenation	0.68
Solid product of hydrogenation in the presence of NiCuMo/SiO2	0.72

) owing to the removal of amorphous cellulose during hydrogenation of the wood in ethanol at 250 °C.

2.6. Composition of Gaseous Products of Abies Wood Hydrogenation

Gaseous products of non-catalytic hydrogenation of abies wood contain mainly carbon oxides and methane. The yield of gases increases to 11.8 wt% in the presence of the NiCuMo/SiO₂ catalyst (Table 1). Carbon monoxide (7.4 wt%) predominates in the gaseous products of the catalytic wood hydrogenation and methane (3.1 wt%) in the products of the non-catalytic hydrogenation. The yield of CO may be increased due to intensification of the reactions of acidic catalytic hydrolysis of ester bonds in lignin. Thus, the formed ketones may undergo decarbonylation on the metal sites of the catalyst to evolve CO [61]. It is probable that methane is formed via hydrocracking of structural aliphatic fragments of lignin and of formed alkyl derivatives of methoxyphenols [62,63].

2.7. Fractionation of Abies Wood

The reductive catalytic fractionation (RCF) of lignocellulose biomass makes it possible to produce low molecular mass products from lignin, while the main part of cellulose is preserved [19,64]. A higher yield of guaiacyl-type monomers (for example, 4-propylguaiacol and 4-propanolguaiacol) is achieved by RCF processing of softwood [20].

Literature data are available on the use of noble metal-based catalysts (Ru, Pt, Pd) for RCF of softwood. For example, Ru/C and Pt/ZrO₂ catalysts were used for RCF of abies wood [16,54] and Pd/C for RCF of pine wood [39].

In the present work, we observed yields of liquid product as high as 42% from the wood weight upon hydrogenation of abies wood in the presence of inexpensive Ni-containing catalyst. Monomer alkyl derivatives of methoxyphenols, mainly 4-propylguaiacol and 4-propanolguaiacol (up to 12 and 16 wt%, respectively) predominate in the liquid products. The data obtained allow us to suggest an inexpensive and widely available NiCuMo/SiO₂ catalyst as an alternative to noble metal-based catalysts for reductive fractionation of abies wood with hydrogen in ethanol medium to produce monomer phenol compounds and cellulose.

Methoxyphenols can be used for the production of epoxy resins and polycarbonates [65,66] in medicine [67], and as components of motor fuels.

The cellulose product obtained by catalytic hydrogenation of abies wood (39.5 wt% yield) contains 73 wt% of cellulose; it may be converted to levulinic acid, glucose and other valuable chemicals [68,69].

3. Materials and Methods

3.1. Preparation of Abies Wood Samples

The abies wood used for the studies contained (per mass of absolutely dry wood) 45.7% cellulose, 25.3% lignin, 17.7% hemicelluloses, 6.2% extractive substances, 0.5% ash. The wood was ground using a vibrobench VR-2 and deresinated with petroleum ether and then acetone (in accordance with the standard method ANSI/ASTM D 1105), then dried at 80 °C to provide less than 1 wt% moisture.

3.2. Catalyst Preparation

NiCuMo/SiO₂ catalyst was prepared by the sol–gel method described elsewhere [70]. Commercial hydrate of basic nickel (II) carbonate, basic copper (II) carbonate and molybdenum (VI) oxide were taken in the required quantities to mix them with aqueous ammonia solution (25% NH₃) and bidistilled water under continuous stirring. The resulting suspension was filtered, dried in air at 120 °C for a night and calcined at 400 °C for 4 h. The mixed nickel–copper–molybdenum oxide systems were fractioned to obtain 2–5 mm particles. Impregnation of the obtained granules with ethyl silicate (32 wt% SiO₂) was followed by calcination at 500 °C for 2 h. The addition of SiO₂ stabilize the active metallic phase during reduction and thermal treatments. This stabilizing agent has no activity in the hydrogenation process. Using SiO₂ as an inert stabilizer for the active component is supposed to reduce the tendency to coke formation [71,72]. Then, the samples were reduced in flowing hydrogen (30 cm³/min·g_cat) at 500 °C (this temperature was chosen according to the TPR data). In a quartz reactor, exposed at this temperature for one hour in hydrogen (15 cm³/min·g_cat), cooled and passivated with a O₂ (2%)/N₂ mixture.

The physical and chemical characteristics of the used NiCuMo/SiO₂ catalyst were studied previously in [73] with the use of a set of physicochemical methods, including temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM).

The texture characteristics of the catalyst were determined from the N₂ adsorption–desorption isotherms at 77 K using a Micromeritics ASAP-2020 Plus instrument (US). The main textural characteristics are given in

Table 7. The composition and characteristics of the catalyst.

Catalyst	Composition, wt%				Mo/Ni Atomic Ratio	SBET, m2/g a	V∑, cm3/g b	Vmicro, cm3/g c	<d>pore, Å d
	Ni	Cu	Mo	Si
NiCuMo/SiO2	46	6.7	11.7	15.0	0.16	109.0	0.23	0.01	83

^a S_BET—specific surface area according to BET; ^b V_Σ—total pore volume; ^c V_micro—micropore volume; ^d <d>—average pore size.

.

The HRTEM images of the fresh and reduced samples show the presence of highly dispersed uniformly distributed metallic particles with an average size of 6.8 nm. All elements (Ni, Cu, Mo, Si) are uniformly distributed in the catalyst [73]. According to XPS data molybdenum in the catalyst is present mainly in reduced Mo⁴⁺ state [73]. XRD data showed that after catalyst reduction and passivation Ni and Cu are present mainly in the reduced metallic state. Additionally, it was found the formation of Ni-based solid solutions with possible incorporation of Cu and/or Mo atoms into the metallic Ni structure [73].

3.3. Hydrogenation of Abies Wood

A 300 mL ChemReSYStem R-201 (Republic of Korea) autoclave was used for wood hydrogenation by the method described elsewhere [16]. First, 50 mL of ethanol, 5.0 g of wood sawdust, 0.5 g of a catalyst were loaded to the reactor. Then, the autoclave was hermetically sealed and blown through with argon to remove air. Hydrogen was fed at the initial pressure of 4 MPa.

The wood was hydrogenated at 250 °C for 3 h under constant stirring at 800 rpm. The temperature was risen at the rate of 10 °C/min to reach the required temperature in 20–25 min. The operation pressure in the reactor was 9.0 MPa. After the reaction mixture was cooled to room temperature, the gaseous products were collected to a gasometer to measure their volume and to determine the composition using gas chromatography. The liquid and solid reaction products were drawn quantitatively with ethanol from the autoclave, and the resulting mixture was separated by filtering.

The solid product was washed with ethanol and dried at 80 °C to constant weight. The liquid product was freed from the solvent using a rotary evaporator at room temperature. The yields of liquid (α₁), solid (α₂) and gaseous (α₃) products were determined by formulae:

$${\mathsf{\alpha }}_1=\frac{{\mathrm{m}}_{\mathrm{l}}\left(\mathrm{g}\right)}{{\mathrm{m}}_{\mathrm{init}}\left(\mathrm{g}\right)}\times 100\%,$$

(3)

$${\mathsf{\alpha }}_2=\frac{{\mathrm{m}}_{\mathrm{s}}\left(\mathrm{g}\right)-{\mathrm{m}}_{\mathrm{KT}}\left(\mathrm{g}\right)}{{\mathrm{m}}_{\mathrm{init}}\left(\mathrm{g}\right)}\times 100\%,$$

(4)

$${\mathsf{\alpha }}_3=\frac{{\mathrm{m}}_{\mathrm{g}}\left(\mathrm{g}\right)}{{\mathrm{m}}_{\mathrm{init}}\left(\mathrm{g}\right)}\times 100\%,$$

(5)

where m_l is the liquid product weight (g), m_init the initial sample weight (g). m_g the gas product weight (g), m_s the solid residue weight (g).

3.4. Thermochemical Properties of Abies Wood

Thermogravimetric analysis was conducted in flowing argon in a temperature range of 3 to 900 °C using a STA 449 F1 Jupiter (NETZSCH) instrument. The heating rate was 10 °C/min. The results obtained were processed using the NETZSCH Proteus Thermal Analysis 5.1.0 program package.

3.5. Composition and Structure of the Products of Abies Wood Hydrogenation

Ethanol-soluble liquid products were analyzed via GC-MS using an Agilent 7890A chromatograph equipped with an Agilent 7000A TripleQuad detector of selective masses at recording the total ion current. Products were separated using a capillary column HP-5MS at temperatures programmed between 40 and 250 °C. Products were identified using the NIST MS Search 2.0 database.

Molecular mass distribution in the liquid products was determined by gel permeation chromatography using an Agilent 1260 InfinityII Multi-Detector GPC/SECSystem chromatograph and refractometric detection at 658 nm. A PLgelMixed-E column with tetrahydrofuran stabilized by 250 ppm butylhydroxytoluene as the mobile phase was used for the separation. The column was calibrated using polydisperse polystyrene standards (Agilent, Santa Clara, CA, USA) and monomer phenol compounds. The eluent was fed at the rate of 1 mL/min, with the sample fed being 100 mL in volume. Samples to be analyzed were dissolved in THF (1 mg/mL) and filtered through a membrane PTFE filter (Millipore). Data obtained were collected and processed using the Agilent GPC/SEC MDS program package.

Composition of gaseous products was GC determined using a Crystal 2000 M chromatograph (Chromatec, Russia) equipped with a thermal conductivity detector at 170 °C, with helium being used as the gas carrier (15 mL/min). Gases CO and CH₄ were analyzed using a column filled with NaX zeolite (3 m × 2 mm) at 60 °C in isothermal mode. A Porapak Q column was used for analysis for CO₂ and hydrocarbon gases in the mode: 60 °C for 1 min then the temperature was risen at the rate of 10 °C/min to 180 °C.

The solid products of abies wood thermal transformation were analyzed for the contents of hemicelluloses, cellulose and lignin. The lignin content was determined using the Kovarov method [74] by hydrolyzing the solid product with 72% sulfuric acid. The content of hemicelluloses was determined by hydrolyzing the solid product with 2% sulfuric acid [75].

IR spectra of the solid products were taken using a Shimadzu IR Tracer-100 spectrometer (Japan, Tokyo) in the wavelength region of 400–4000 cm⁻¹. The spectral data were processed using the OPUS (version 5.0) program package. Solid samples to be analyzed were prepared in the form of pellets in KBr matrices (2 mg of sample/1000 mg KBr).

XRD studies of the solid products were carried out using a DRON-3 X-ray diffractometer with monochromatized CuKα irradiation (λ = 0.154 nm) at 30 kV voltage and 25 mA current. Sampling interval was 0.02° step, in 1 s intervals per data point. Measurements were performed in the Bragg angle 2Θ region from 5.00 to 70.00Θ.

Crystallinity index (CI) was calculated from the ratio of the height between the intensity of the crystalline peak (I₀₀₂ − I_AM) to total intensity (I₀₀₂) after subtraction of the background signal:

$$CI=\frac{I_{002}-I_{AM}}{I_{002}},$$

(6)

where I₀₀₂ is the height of peak 002; I_AM is the height of the minimum between peaks 002 and 101 [76].

Elemental compositions of the wood, liquid and solid products of wood transformation were determined using a HCNS-O ThermoQuest Flash EA-1112 analyzer (Waltham, MA, USA).

4. Conclusions

The NiCuMo/SiO₂ catalyst is suggested for use for the hydrogenation of abies wood with hydrogen in the ethanol medium at 250 °C. The catalyst provides an intensification of the reactions of reductive depolymerization of lignin and deoxygenation of the resulting organic compounds, which leads to an increase in the yield of liquid products up to 42 wt% and a decrease in the O/C ratio in these products, compared to the non-catalytic process.

GPC studies showed that the mass-average molecular mass (1370 Da) and polydispersion (2.66) of the liquid products of catalytic hydrogenation of abies wood are lower (due to the increased contents of monomer and dimer compounds) than those of the liquid products of non-catalytic hydrogenation (1870 Da and 3.01, respectively).

GC-MS studies of the liquid products of abies wood hydrogenation demonstrated that the yield of monomer methoxyphenols with predominant contents of 4-propylguaiacol and 4-propanolguaiacol is three times increased (up to 36.8 wt%) in the presence of a catalyst.

According to IRS, XRD, elemental and chemical analysis data, the composition and structure of the solid product of abies wood hydrogenation corresponds to cellulose-I. Carbon monoxide predominates among the gaseous products of the catalytic hydrogenation and CH₄ among the products of the non-catalytic hydrogenation. Probably, the former is formed via decarbonylation and the latter via hydrogenolysis reactions.

The study undertaken revealed the possibility of substituting the expensive Ru-, Pt-, and Pd-based catalysts for the cheaper NiCuMo/SiO₂ catalyst in the process of reduction catalytic fractionation of abies wood into cellulose and liquid products with high contents of 4-propylguaiacol and 4-propanolguaiacol.